The display apparatus, such as a television picture tube or a display tube, comprises an evacuated envelope which includes a faceplate and a rectangular funnel. A tri-color luminescent screen is disposed on the interior surface of the faceplate. An electron gun for generating and directing at least one, and preferably three electron beams toward the screen is disposed within and closes the neck of the funnel. The panel, bearing the screen, is sealed to and closes the opposite end of the funnel. The interior surface of the funnel is coated with a first conductive layer and the exterior surface is partly covered with a second conductive layer. The conductive layer on the interior surface of the funnel forms the connection between the last electrode of the electron gun and the screen on which the electron beams impinge. The conductive layer on the interior surface of the funnel thus acts as a second anode and also creates a field-free space in the region of the tube in which the electron beams are not deflected. The conductive layer on the exterior of the funnel is grounded and, together with the interior conductive layer, forms a capacitor which serves to smooth the applied high voltage. The apparatus further includes a system of deflection coils, called a deflection yoke, disposed around the neck-funnel transition region of the envelope.
It is known in the art to provide a high resistance coating or layer on the exterior portion of the tube envelope underneath the deflection yoke, in order to reduce interference generated by the deflection system. Typically, the high resistance layer has a resistance of between 10.sup.6 and 10.sup.10 ohms per square. The high resistance layer is electrically connected to the second conductive layer disposed on the exterior surface of the funnel. Apparatuses made with a low resistance layer, or having no coating at all underneath the yoke, develop an inductive charge which "flashes-over", or electrically discharges, to the deflection yoke or to the second conductive layer on the exterior surface of the funnel. The discharge is accompanied with an annoying crackling or sputtering sound.
The problem is exacerbated in the current generation of high resolution display apparatuses in which the horizontal scan rate is twice that of prior tubes, thus requiring the deflection yoke to operate at a higher current, thereby generating additional heat underneath the yoke and increasing the magnitude of the induced charge. The high resistance coating underneath the deflection yoke also must be compatible with other steps in the manufacturing process, such as alignment and attachment of the deflection yoke. In the present manufacturing process, the yoke, after alignment, is rapidly secured by an adhesive to the tube. Conventional high resistance coatings, such as those known in the art and containing polyvinyl acetate and suitable quantities of metal oxides and soots, or water-based urethane resins and conductive materials, are unsatisfactory, because they are either incompatible with, or slow, the curing process of the adhesive. Thus, a need exists for a high resistance coating which is compatible with the above-described yoke attachment process, and which provides the protection needed to reduce electrical interference from the yoke and to prevent the build-up and flash-over of induced charge.